1. Field of the Invention
The present invention relates generally to cryogenic refrigerators, and more particularly to a cryogenic refrigerator including a displacer.
2. Description of the Related Art
Gifford-McMahon (GM) refrigerators have been known as cryogenic refrigerators that include a displacer. In the GM refrigerator, the displacer is caused to reciprocate in a cylinder by a drive unit.
In such an environment, an expansion space is formed between the cylinder and the displacer. The displacer reciprocates in the cylinder to expand a high-pressure refrigerant gas fed into the expansion space, thereby producing cryogenic temperatures.
In general, in this type of GM refrigerator, during one cycle of reciprocation in a cylinder, the speed at which the displacer moves from the top dead center to the bottom dead center is equal to the speed at which the displacer moves from the bottom dead center to the top bottom center. That is, conventionally, during one cycle of its reciprocation, the displacer is designed to move in a cylinder so that when plotted, its movement is along a substantial sine wave. (See, for example, Japanese Patent No. 2617681.)